Basic power steering systems provide a fixed steering assist offset curve regardless of driving conditions. Some recently appearing power steering systems, however, provide apparatus capable of changing power steering effort in different driving situations. For example, one such system, marketed by Delphi Automotive Systems, provides a decrease in steering assist with vehicle speed, so that great assist will be provided at very low speed vehicle maneuvers such as parking, while significantly less assist is provided at high speed highway driving for a tighter, "manual steering feel" and greater stability. In a recent variation of this system, a variation responsive to estimated lateral acceleration has been added to improve steering linearity.
Such improvements enhance vehicle steering feel and performance during normal handling of the vehicle, when there is good friction between the road surface and vehicle tires. But handling near or at the limit, when road surface friction is significantly decreased, may often require a different adjustment in steering assist.